EGR has been typically known as a technique to reduce nitrogen oxides (NOx) contained in exhaust gas of a diesel engine. In EGR, a part of the exhaust gas of the engine is returned to an intake pipe from an exhaust pipe through an EGR pipe. At this time, an EGR valve device is provided on the EGR pipe or on a branch portion of the exhaust pipe and the EGR pipe. By controlling a valve body included in the EGR valve device to open and close the valve body in accordance with an operating condition of the engine, a proper amount of the exhaust gas is returned to the intake pipe at a proper timing.
The valve body of the EGR valve device is biased toward a closing side by a valve spring. A base end of the valve body is connected to a hydraulic piston. Accordingly, when the valve body is opened, the hydraulic piston is moved by hydraulic pressure against biasing force of the valve spring, pressure of exhaust gas, or hydrodynamic force of exhaust gas to lift the valve body (see Patent Document 1) By balancing the hydraulic pressure with the biasing force and the pressure and hydrodynamic force of the exhaust gas, the opening degree of the valve body is maintained.    Patent Document 1: International Publication No. 05/095834